Some window structures mounted in buildings or vehicles to separate the indoor space and the outdoor space employ a multiple-glazed glass unit. The multiple-glazed glass unit includes a pair of glass panes opposed across a gap layer to be spaced at a predetermined distance from each other. Thanks to the presence of this gap layer, the window structure employing the multiple-glazed glass unit has better heat insulating properties than a window structure employing a single glass pane. The improvement in heat insulating properties of the window structure can provide a reduction in the energy required for indoor air conditioning.
In recent years, a multiple-glazed glass unit employing a glass pane having a principal surface (glass surface) on which a heat shielding film is formed has been developed in an attempt to provide heat shielding properties in addition to heat insulating properties. With the use of a window structure additionally having heat shielding properties, entry of strong sunlight into an indoor space can be reduced during daytime, and inflow of heat from an outdoor space can be decreased during night, particularly in summer. A known example of the heat shielding film is a low-emissivity film (Low-E film). For example, JP 2013-517206 A discloses a multiple-glazed glass unit including a first glass pane facing an outdoor space and a second glass pane facing an indoor space, the second glass pane having an external surface (a principal surface facing the indoor space; this surface corresponds to a fourth face (face #4) described later) on which a low-emissivity film including a transparent electrically-conductive film is formed. JP 2013-517206 A also discloses that in this glass unit, a low-emissivity film including a film containing silver may further be formed on an internal surface (a principal surface facing a gap layer; this surface corresponds to a second face (face #2) described later) of the first glass pane.